The present invention relates to methods of inspecting integrated circuit substrate, and more particularly to methods of inspecting integrated circuit substrates at intermediate stages of fabrication.
As a design rule gets smaller in fabricating semiconductor devices, it becomes very difficult to form a contact hole which opens a predetermined portion of a semiconductor substrate. Accordingly, it is important to monitor whether or not a contact hole is formed properly on a semiconductor substrate. In particular, by effectively monitoring in-line the state, open or not-open, of a contact hole during fabrication of a semiconductor device, the time required for fabricating the semiconductor device can be reduced and the yield can be greatly enhanced.
However, conventional in-line monitoring of an open or not-open state of a contact hole has been performed manually by an operator using a general image measurement apparatus. In this case, a monitoring error may result from the manual operation and thus unreliable results may be obtained.
Also, in order to monitor the open or not-open state of a contact hole using the general image measurement apparatus, a high voltage of about 20 KV must be used. However, if a high voltage such as about 20 KV is used, it is not possible to precisely monitor the state, open or not-open, of a contact hole.
FIG. 1 is a schematic diagram illustrating a conventional electron beam inspection apparatus, and FIG. 2 illustrates a semiconductor substrate used for determining whether a contact hole is in an open or not-open state. In detail, the electron beam inspection apparatus shown in FIG. 1 includes an electron-beam gun 1 for supplying electron beams. The electron beams are emitted from the electron-beam gun 1 and are accelerated before passing through a gun aperture 3. The electron beam inspection apparatus includes electron beam receiving means which allows an electron beam to move properly toward a sample, e.g., the surface of a semiconductor substrate, for scanning the semiconductor substrate. Here, a condenser lens 5 collimates the diverging electron beam 2 and the collimated electron beam 2 passes through an electrostatic octapole 7 for astigmatism correction and alignment. Then, the electron beam 2 having passed through the electrostatic octapole 7 passes through a beam adjusting aperture 9 and an icosapole deflector 11. The electron beam 2 having passed through the beam adjusting aperture 9 and the icosapole deflector 11 passes through an objective lens 13 to then be incident on a sample 15 shown in FIG. 2. Either a positive voltage or a negative voltage may be applied to the sample 15 in the electron beam inspection apparatus. An x-y stage (not shown) capable of moving in x and y axes directions and supporting the sample 15 is provided under the sample 15.
In the electron beam inspection apparatus, secondary electrons emitted from the sample 15 after passing through the objective lens 13 are detected by a secondary electron detector 21 via an extraction electrode 17 and a Wien filter 19. A positive voltage is applied to the extraction electrode 17 and a negative voltage is applied to the sample 15 so that the secondary electrons move to the secondary electron detector 21 via the Wien filter 19 to then be detected. The Wien filter 19 comprised of an electrostatic octapole and a 60xc2x0 magnetic field deflector, removes opposed electric and magnetic deflections for the electron beam 2. The sample 15 may be in the form of a semiconductor substrate 31 shown in FIG. 2, or a mask substrate (not shown). If the sample 15 is a mask substrate having an electron beam transmission area, the electron beam 2 is received by a transmitted electron detector 25 via an electrostatic transmission lens 23 to then be detected.
The possibility of determining in-line the state, open or not-open, of a contact hole will now be evaluated in the case of testing the sample semiconductor substrate shown in FIG. 2 with the electron beam inspection apparatus shown in FIG. 1. First, the sample semiconductor substrate shown in FIG. 2 tested with the electron beam inspection apparatus shown in FIG. 1 will be described. In detail, a gate oxide layer (not shown), a gate electrode consisting of a polysilicon layer 33 and a tungsten silicide layer 35, and an insulation layer 37 for insulating the gate electrode are sequentially formed on a p-type silicon substrate 31 having an n-type impurity region 32 to be a source and a drain. Spacers 39 are formed at both side walls of the stacked structure of the gate electrode and the insulation layer 37. A contact hole 41 which exposes the silicon substrate 31 between the spacers 39 is formed. Next, in-line monitoring of whether a contact hole formed in the semiconductor substrate shown in FIG. 2 is in an open or not-open state is performed using the electron beam inspection apparatus shown in FIG. 1. Here, if an unetched material layer 43 shown in FIG. 2 (e.g., an oxide or nitride layer) is present in the contact hole, primary electrons (represented by reference numeral 45 in FIG. 2) do not flow properly to the silicon substrate 31 so that electrons accumulate on the surface of the unetched material layer 43. Then, a large amount of secondary electrons (represented by reference numeral 47 in FIG. 2) are emitted from the surface of the silicon substrate 31 due to repulsive force of electrons. Depending on a difference in secondary electron yields, a brighter (white) or darker (black) image is displayed for a portion where a large amount of secondary electrons 47 are emitted, that is, a portion where the unetched material layer 43 is present, compared to portions where the unetched material layer 43 is not present.
Notwithstanding this method of performing in-line inspection, it may still be difficult to reliably detect the presence of unetched material. For example, FIG. 3 illustrates movement of primary and secondary electrons when performing in-line monitoring of a contact hole using the electron beam inspection apparatus shown in FIG. 1. In FIG. 3, the same reference numerals as those of FIG. 2 denote the same elements. In detail, the primary electrons 45 incident on the substrate 31 are activated within the unetched material layer 43 in the contact hole and in the substrate 31 and then move toward the lower portion of the substrate 31. Then, since the difference in secondary electron yields between a portion with the unetched material layer 43 and a neighboring portion without the unetched material layer 43 is not large, the bright and dark sections of the image are not as distinguishable. Thus, it may be difficult to inspect the state, open or not-open, of the contact holes shown in FIGS. 2 and 3, by using the electron beam inspection apparatus shown in FIG. 1.
FIGS. 4 through 6 illustrate images of contact holes observed by a high-voltage electron beam inspection apparatus in the case when the unetched material layer shown in FIG. 3 is an oxide layer. In detail, FIGS. 4A through 4C show that the unetched material layer 43 shown in FIG. 3 (i.e., an oxide layer) is removed by etching. Specifically, in view of a flat zone of a semiconductor wafer, FIG. 4A shows a cell portion present on the top of the semiconductor wafer, FIG. 4B shows a cell portion in the center thereof, and FIG. 4C shows a cell portion at the bottom thereof. In these cases, the oxide layer on the edge portion of the cell is not completely etched, thus preventing the contact hole from being opened, so that a bright image is displayed. The oxide layer of the interior portion of the cell is completely etched, so that a dark image is displayed. FIGS. 5A through 5C show cases where the thickness of the unetched material layer 43 is 300 xc3x85. Here, like in FIGS. 4A through 4C, in view of a flat zone of a semiconductor wafer, FIG. 5A shows a cell portion on the top of the semiconductor wafer, FIG. 5B shows a cell portion in the center thereof, and FIG. 5C shows a cell portion at the bottom thereof. In these cases, a bright image is displayed on the edge portion of the cell, and a dark image is displayed on the interior portion of the cell. Even if the thicknesses of the oxide layers shown in FIGS. 5A through 5C are set to 300 xc3x85, the oxide layers shown in FIGS. 5A through 5C are not discernible from those shown in FIGS. 4A through 4C. Thus, the state (open or not-open) of a contact hole cannot be determined. FIGS. 6A through 6C show cases where the thickness of the unetched material layer 43 shown in FIG. 3 is 500 xc3x85. Similarly, in view of a flat zone of a semiconductor wafer, FIG. 6A indicates a cell portion on the top of the semiconductor wafer, FIG. 6B indicates a cell portion in the center thereof, and FIG. 6C indicates a cell portion at the bottom thereof. In these cases, a bright image is displayed on the edge portion of the cell, and a bright image is displayed on the rim of a contact hole in the interior portion of the cell, which suggests that the contact hole is not opened. Therefore, if the thickness of the unetched material layer 43 as the oxide layer is set to 500 xc3x85, it can be determined whether a contact hole is in an open or not-open state. Thus, in order to reliably inspect the state of a contact hole using the electron beam inspection apparatus shown in FIG. 1, the thickness of the unetched oxide material layer 43 must be at least 500 xc3x85.
FIGS. 7 and 8 illustrate images of contact holes observed by a high-voltage electron beam inspection apparatus in the case when the unetched material layer shown in FIG. 3 is a nitride layer. In detail, FIGS. 7A through 7C show images of contact holes observed by a high-voltage electron beam inspection apparatus in the case where the unetched material layer shown in FIG. 3 is a nitride layer. Here, in view of a flat zone of a semiconductor wafer, FIG. 7A indicates a cell portion on the top of the semiconductor wafer, FIG. 7B indicates a cell portion in the center thereof, and FIG. 7C indicates a cell portion at the bottom thereof. In these cases, the nitride layer is not completely etched at the edge portion of the cell, so that the contact hole is not opened, thereby causing a bright image to be displayed. In the interior portion of the cell, the nitride layer is completely etched, thereby displaying a dark image.
FIGS. 8A through 8C show images of contact holes observed by a high-voltage electron beam inspection apparatus in the case where the unetched nitride material layer 43 is 300 xc3x85 thick. Here, in view of a flat zone of a semiconductor wafer, FIG. 8A indicates a cell portion on the top of the semiconductor wafer, FIG. 8B indicates a cell portion in the center thereof, and FIG. 8C indicates a cell portion at the bottom thereof. In these cases, a bright image is displayed at the edge portion of the cell, and a dark image is displayed in the interior portion of the cell. That is to say, the contact hole is in a xe2x80x9cnot-openxe2x80x9d state. Therefore, it can be determined whether a contact hole is in an open or not-open state when the thickness of the nitride layer is set to 300 xc3x85.
Notwithstanding the ability to reliably detect the presence of unetched material in contact holes when that material is of sufficient thickness (at least 500 xc3x85 for oxide and at least 300 xc3x85 for nitride), there continues to be a need to more reliably detect the presence of unetched material in contact holes when the unetched material is thinner than 300 xc3x85.
It is therefore an object of the present invention to provide improved methods of inspecting integrated circuit substrates.
It is another object of the present invention to provide methods of detecting the presence of insulating residues in contact holes on integrated circuit substrates.
These and other objects, advantages and features of the present invention are provided by methods of inspecting integrated circuit substrates (e.g. wafers) to determine whether unwanted insulating residues remain in contact holes thereon, after etching or related fabrication steps have been performed. These integrated circuit substrate inspection methods preferably include the steps of directing a beam of electrons into a first conductive plug located within a first contact hole on an integrated circuit substrate and then measuring a quantity of electrons emitted from the first conductive plug to determine an absence or presence of an electrically insulating residue in the first contact hole. In particular, the quantity of electrons emitted from the first conductive plug by secondary electron emission can be measured in order to determine whether electrons are being accumulated within the conductive plug because an insulating residue is blocking passage of the electrons into an underlying conductive portion of the substrate. If an electrically insulating residue is present, then sufficient repulsive forces between the accumulated electrons will result in the secondary emission of excess electrons from an upper surface of the conductive plug as the conductive plug is being irradiated with the electron beam. A detector can then be used to measure the quantity of the emitted electrons against a threshold level, in order to determine whether the quantity of electrons emitted by secondary emission is sufficient to indicate that an insulating residue is present in the contact hole.
The reliability of these methods to determine whether insulating residues remain within contact holes after etching can also be increased by annealing the integrated circuit substrate and etching back the electrically insulating layer in which the contact holes are formed to more fully expose the conductive plugs within the contact holes. This etching step preferably comprises the step of exposing the electrically insulating layer and the conductive plugs therein to an etching solution comprising HF and NH4OH. The annealing step may also comprise the step of annealing the integrated circuit substrate at a temperature greater than about 800xc2x0 C. for a duration greater than about one (1) hour. In order to increase the rate at which electrons within the electron beam are transferred from the conductive plugs into the conductive portion of the substrate, a voltage of greater than 100 volts is preferably applied across the semiconductor wafer.
According to another embodiment of the present invention, a method of inspecting an integrated circuit substrate having upper and lower electrically insulating layers stacked thereon may include the steps of directing a beam of electrons into a first conductive plug located within a first contact hole in the upper electrically insulating layer and measuring a quantity of electrons emitted from the first conductive plug to determine an absence or presence of an electrically insulating residue in a second contact hole in the lower electrically insulating layer. Thus, even during back-end processing after multiple insulating layers have been formed on an integrated circuit substrate, the methods of the present invention can be utilized to detect the presence of insulating residues formed in contact holes of underlying insulating layers.